This invention relates in general to electrophotography and more specifically, to an improved electrophotographic imaging member having an a more sensitive charge generating layer.
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer in either a flexible belt form or a rigid drum configuration. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side of the electrically active layers, to render the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In U.S. Pat. No. 4,265,990 a layered photoreceptor is disclosed having separate charge generating (photogenerating) and charge transport layers. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive material may be formed as a continuous, homogeneous photogenerating layer. Many suitable photogenerating materials known in the art can be utilized, if desired.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems employed flexible photoreceptor belts, operating at very high speeds, have also placed stringent mechanical requirements and narrow operating limits as well on photoreceptors. Advanced photoreceptors have excellent electrical and mechanical properties. Some have very stable electrical performance over long life, for example, up to at least 200K cycles. However, many photoreceptors exhibit fluctuations in photosensitivity from batch to batch even where every effort is made to ensure identical processing conditions such as the milling of charge generation layer pigment dispersion under the same conditions. For example, when extrinsic photosensitive pigments are employed, the photogenerated carriers must be brought out of the surface of pigment particles before the carriers recombine and move into the charge transport layer under the applied electric field. This process slows down considerably in binders containing dispersed extrinsic photosensitive pigment particles such as benzimidazole perylene particles, especially at low applied electric fields. Under these conditions, the photoinduced discharged curve (PIDC) becomes softer at low field. Such a soft PIDC curve requires more powerful, bulky and expensive laser light sources for imaging in an electrophotographic printer or duplicator. The expression photoinduced discharged curve (PIDC) as employed herein is defined as a relationship between the potential as a function of exposure and is a measure of the sensitivity of the device. It generally represents the supply efficiency (number carriers injected from the generator layer into the transport layer per incident photon) as a function of the field across the device.